Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 142322

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
622 PM CDT Mon Aug 14 2017

Showers now pushing into southwest WI are lined up along a weak
surface trough that will drift across southern Wisconsin this
evening. Given the overall lack of good instability around here,
these should stay mostly showers, but an isolated thunderstorms is
possible. Interestingly, there is enough low level shear to cause
them to rotate. A few funnels have been reported in far eastern
IA. This should settle down as they move east and we lose the


There is a small chance of some rain at the TAF sites overnight as
a weak surface trough pushes across southern Wisconsin. Look for
VFR conditions through midnight, then weak flow and abundant low
level moisture should lead to widespread MVFR and IFR conditions
from about 07z through 15z Tuesday. VFR conditions return by
afternoon as we readily mix out.


.PREV DISCUSSION... (Issued 330 PM CDT MON AUG 14 2017)

TONIGHT AND TUESDAY - Forecast Confidence...Medium trending to High.

Can not rule out a few showers and weak thunderstorms affecting
southern Wisconsin late this afternoon and tonight.  Water vapor
imagery and VAD network show mid-level trof over northwest WI
extending south into southeast MN.  This trof wl sweep across WI
overnight with the best chance for scattered activity across
northeast CWA closer to wave.  Also reinvigorating upper jet over
central WI will cause right entrance region to affect CWA this
evening.  Soundings show weak elevated CAPE up to 500 j/kg through
the evening.  Precipitation chances will decrease from west to east
overnight as short wave moves off to the east.  Weak convergence
along passing front may bring some areas of low clouds and fog to
the area later tonight.

Broken to overcast low clouds lingering into Tuesday morning should
become scattered through the day as drier air aloft mixes down and
better subsidence from passing short wave ridging.

TUESDAY NIGHT THROUGH THURSDAY - Forecast Confidence...Medium.

Much better agreement between short term guidance regarding passing
short wave trof later Wednesday into Wednesday night. ECMWF has
significantly slowed down speed of passing wave while NAM has come
into better agreement on strength of system.  All the guidance has
slowed down the eastward progression of the system.  With drier air
hanging on, pulled back on pops Tuesday night.  Increasing warm air
and moisture advection Wednesday morning as upper level jet begins
to slide eastward into western WI.  Chances for showers and storms
will spread across the area on Wednesday with the most favored time
for more widespread showers and storms from late afternoon through
06Z Thursday when best synoptic and mesoscale forcing is expected.
Secondary short wave rotating around upper low wl continue to bring
showers to the area along with isolated storms on Thursday.  Any
showers and storms may bring a period of heavy rainfall to the area
Wednesday evening as impressive pwat values above 1.5 inches is
expected and forward propagating vectors not overly impressive
Wednesday afternoon.  Warm cloud depth will be a bit lower around 10-
10,500 ft.


THURSDAY NIGHT THROUGH MONDAY - Forecast Confidence...Low to Medium.

A quiet start to the extended period is expected as short wave
ridging moves across the western Great Lakes.  The quiet conditions
should last through most of Friday night, however large differences
in the medium range guidance show by this time.  GEM and ECMWF have
been deamplifying approaching short wave in northwest steering winds
for this period but 00z-06z GFS holding on to strong trof moving
through WI on Saturday.  Most GFS ensemble perturbations show weaker
low pressure trof and several even show lingering high pressure
through 12Z Saturday.  WPC deemphasized the 06Z GFS in its model
blend focusing more on the comparable GEM and ECWMF ensemble.  Hence
wl continue dry conditions for this period.

Upper level steering flow wl transition to more zonal to
southwesterly winds later in the weekend and continue into early
next week.  Some of the medium range guidance is showing increasing
warm air advection triggering a few showers or storms Sunday into
Monday but forecast soundings look fairly dry with elevated warm
layer preventing updraft initiation.  Hence for now thinking most of
this period will be dry, but may be keeping in 20=30 percent pops
for now.

At this point, still think high clouds could be a factor in eclipse
viewing early Monday afternoon from convection located to the south.
Also expect some cumulus but should be few-sct.  Temperatures likely
to be slightly below seasonal normal on Friday but should warm to
near or above normal by the end of the period.

AVIATION(21Z TAFS)...Lower VFR sct-bkn cigs wl affect TAF sites
through the evening.  Still can not rule out a shower or
thunderstorm into the evening as upper trof and surface front moves
through.  Weak convergence along front still expected to generate
areas of lower clouds later tonight along with patchy fog.

MARINE...Southeast winds will turn to the north to
northeast later tonight into Tuesday as a surface front moves
through. Winds are expected to remain less than 20 knots through
Tuesday. Strongest winds likely to be Wednesday into Thursday as low
pressure passes through the western Great Lakes.




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